1. Field of the Invention
The present invention relates to integrated circuits. More particularly, the present invention relates to programmable logic devices and to Field programmable Gate Array (FPGA) integrated circuits.
Field Programmable Gate Arrays (FPGAs) are typically used in secure systems because the design is implemented by the user not the wafer manufacturer, making it more difficult for an attacker to understand the internal function programmed into the device. However, increasingly sophisticated techniques have been developed to determine the design that has been programmed into the FPGA. One of these techniques involves shining a laser through the backside of the die to sense the effect on the transistors.
2. Description of Related Art
Laser light stimulation of transistors has been used for many years to analyze integrated circuits. However, modern integrated circuits utilize very dense metal with as many as 12 metal layers. This prevents light from reaching the transistors from the top. As a result, techniques have been developed to shine light from the backside of the integrated circuit and therefore must use infrared light, as silicon is opaque to visible light, but transparent to infrared light. The infrared light must be reasonably intense to stimulate the transistor due to absorption by the bulk silicon and to be strong enough to cause a measurable effect on the transistor to analyze its function.
There are two different phenomena that can be used in this analysis. First, the infrared light can be used to heat the transistor significantly (tens of degrees Celsius) thereby slowing it down which can be seen as a change in the power consumption at the specific time that it switches, thereby giving clues to an attacker concerning what is happening in the circuit. Second, infrared light can be used to switch the state of a register by inducing a photo current by double photon absorption, thereby altering the decision made by the circuit. The state change of the register can be sensed by the attacker. In the event that the register contains a bit of an encryption key, the attacker obtains one of the bits of the key. By repeating the process, an attacker can read many or all bits in the encryption key thus gaining access to the internal configuration information for the circuit. In other cases, flipping the state of a register may enable readout of otherwise unavailable information, thus potentially giving the attacker access to the configuration information for the integrated circuit.
There is no prior art known to the inventor for dealing with this problem.